i will be arriving late(8pm) at chennai airport on 6 november. It seems there are restrooms at the domestic termimal, which is close by the international. Has anynoe ever stayed in them, and is it possible for an traveller arriving late at the international terminal with plans to catch a train the next day to stay in the rest rooms?

If the restrooms are run the same way as in Delhi then you have to have a departing ticket within next 24 hours before you are considered for a bed!!
Delhi is pretty much always booked out I imagine Chennai would be the same!!
I agree with nick it's not so late just get yourself into Chennai and find a hotel!!
In fact the hotels In Mahaballipuram would still be an option by taxi!!